James,
I replaced my genoa sheets two years ago. Here are a few of the things I considered:
Size
My boat has upgraded 1980s Barient 32 self tailing primary winches. They were not designed for modern, small diameter, high tech lines. I checked them and 9/16ths is the smallest line that will stay in the jaws. Non self-tailers will not have this issue so you can size the sheets by how easy it is on your hands, load and weight. High tech lines will be smaller in diameter, lighter per foot for the same stretch and cost more.
Material
My headsails are newer laminate (Quantum Fusion M) load path sails that do not stretch much. Generally, you would match high tech, low stretch lines to these type sails. Since my sheets are so oversized to fit the winches, I am barely loading them and the stretch of even polyester Sta-Set is minimal at load load. Since my sails are cut to fit on my furler, the sheets are exposed to UV all season. I went with Sta-Set to minimize cost of replacing them oftem. The only downside to Sta-Set and other "low tech" lines is that water absorption is higher than high tech. This weight is tiny and totally not an issue on our boats. If you primarily use a specialized light wind sail or drifter, I would consider the light, high tech line with a cover added on the winch end to bulk it up if needed for winch jaws. I use a 155 genoa as my primary light air headsail and the big, heavy sheets have not been a problem. If you have dacron sails, old dacron sails or don't really know/care then the sheet material becomes even less important.
Length
Measure them carefully, then give yourself a cushion. I had some old sheets to use as a baseline and they were never too short. Mine could be shorter, but I don't mind the extra line. Just make sure you've tried all possible configurations you might use: for example big genoa poled out on giant pole, etc. so you don't come up short. The size of your jib (100 -150 or more?), proportions of your rig (long J vs boat length) and hardware layout (I have turning blocks aft of my genoa tracks that add sheet length) will determine how long the sheet needs to be. Anything unusual about how you sail your boat or the cockpit layout? Does someone like to work the sheets from far away from the winch? Work both sheets at the same time from behind the wheel rodeo style while the autopilot tacks? Hate extra line laying in the cockpit? All these will impact the length.
Mark
